1. Field of the Invention
This invention relates to a video camera device and, in particular, to a video camera device having an exposure control circuit.
2. Description of the Prior Art
There is known a video camera device using an inner focus lens assembly (lens block) as its lens system whose front lens is fixed and whose zoom lens and focus lenses are supported for movements in the lens assembly.
FIG. 1 shows a block diagram of a video camera device using such an inner focus lens assembly. In FIG. 1, an image entering through the inner focus lens assembly 51 is focused on a light detecting surface of a CCD image pickup device 53 through an iris 52. Opening and closing of the iris 52 is controlled in response to an output from an iris driving circuit referred to later. An output signal from the CCD image pickup device 53 is supplied to an AGC amplifier 54 whose gain is controlled in response to an output signal from a D/A converter described later.
An output signal from the AGC amplifier 54 is converted from an analog signal to a digital signal by an A/D converter 55. The digital signal is supplied to a signal processing circuit 56 and to a detecting circuit 57. The signal processing circuit 56 performs predetermined signal processing on the digital signal supplied thereto, and then outputs it from an output terminal 58. The detecting circuit 57 detects the digital signal, and then supplies it to a comparator 59. A reference level setting circuit 60 is connected to the comparator 59 to set a reference level. An output signal from the comparator 59 is supplied to a control amount computing circuit 62 through a loop filter 61.
The control amount computing circuit 62 calculates a desired opening amount for the iris 52 and a desired gain for the AGC amplifier 54 based on the image signal level. Thereafter, an output from the control amount computing circuit 62 is supplied to an iris driving circuit 63 and to a D/A converter 64. Output from the iris driving circuit 63 is supplied to the iris 52 to control the opening amount thereof. The output signal from the control amount computing circuit 62 is also converted to an analog signal by a D/A converter 64 and is supplied to the AGC amplifier 54 to control the gain thereof.
FIG. 2 shows an arrangement for the inner focus lens assembly 51. In FIG. 2, a first group lens F1 and a third group lens F3 of the inner focus lens assembly 51 are fixed while a second group lens F2 and a fourth group lens F4 can move. The second group lens F2 behaves as a zoom lens, and the fourth group lens F4 as a focus lens. The iris 52 is located between the zoom lens and the third group lens F3, and a color separating prism P is located between the focus lens and the CCD image pickup device 53.
Existing video camera devices involve the problem of F drop phenomenon. With reference to FIG. 3, F drop phenomenon is explained. In FIG. 3, incident light through the first group lens F1 is diverged by the zoom lens, and travels through the iris 52. After corrected in direction by the third group lens F3, the light is focused by the focus lens F4 to make an image on the CCD image pickup device 53.
FIG. 3A shows a beam of light travelling when the zoom lens is located at its wide end. Among the incident beam of light d, the beam of light that passes through the iris 52 has a diameter as large as the maximum open area of the iris 52. That is, the quantity of light passing through the iris 52 can be determined by changing the open area of the iris 52. FIG. 3B shows the beam of light travelling when the zoom lens is located midway between its wide and telephoto ends. The quantity of light passing through the iris 52 can be determined by changing the open area of the iris 52 as in FIG. 3A. FIG. 3C shows the beam of light travelling when the zoom lens is located at its telephoto end. In FIG. 3C, the diameter of the beam of light travelling through the iris 52 is smaller than the diameter of the maximum open area of the iris 52. Note that the diameter of the beam of light passing through the iris 52 is determined by the diameter of the first group lens F1.
The F drop phenomenon is that, because of shading of the beam of light caused by the first group lens F1, the diameter of the beam of light passing through the iris 52 becomes smaller than the maximum open area of the iris 52, and makes it impossible to vary the quantity of light even by changing the open area of the iris 52 (J in FIG. 3C).
FIG. 4 shows a gain curve A for the iris and a gain curve B for the AGC amplifier relative to the brightness of a subject in the existing video camera device. The abscissa indicates the quantity of light of the subject while the ordinate indicates the gain of the AGC amplifier. These gain curves are obtained by: EQU Q.times.R.times.S.times.T=U (1)
where Q is the brightness of the subject, R is the gain of the iris, S is the gain of the AGC amplifier, T is the photoelectric conversion ratio of the CCD image pickup device, and U is an output of the detecting circuit.
In equation (1), if the brightness of the subject is in a range in which exposure can be controlled, then the output of the detecting circuit is constant. However, if the F drop phenomenon occurs because of the position of the zoom lens in the inner focus lens assembly 51, it causes an iris blind zone (shown by E) which is a difference (F drop amount F) between the gain of the iris at the wide end (shown at position C) and the gain of the iris at the telephoto end (shown at position D), and the brightness is not changed even by opening and closing the iris. Moreover, equation (1) is not effective in the iris blind zone E, and the effectiveness of the iris decreases. Further, an unnaturalness is produced at the changeover between control by the iris and control by the AGC amplifier (shown at position G). The foregoing statement is indicated by: EQU Q.times.V.times.S.times.T=U (2)
where V is the actual gain of the iris, and by: EQU R-W=V (3)
where R is the same as that of equation (1) and W is a gain corresponding to the F drop amount F.
If the first group lens has a larger diameter, then the F drop phenomenon would be removed. However, the use of larger lenses as the first group lens increases the scale and weight of the lens assembly itself. It does not meet the requirement of a decrease in scale and weight of video camera devices and invites an increase in the cost.
In addition, in case of the aforementioned video camera device in which the exposure control circuit controls the opening amount of the iris and the gain of the AGC amplifier in response to the image pickup signal level, an increase in the amount of F drop increases the region in which the image signal level cannot be changed even by controlling the open amount of the iris. As a result, unnaturalness is produced in the effect of iris variance at the changeover between the iris and the AGC amplifier control, and the performance of the circuit as an automatic exposure control circuit is significantly lowered.